Filter elements can be used to remove contamination in a variety of applications. Such elements can include one or more fiber web(s). The fiber web(s) provide a porous structure that permits fluid (e.g., gas, liquid) to flow through the element. Contaminant particles contained within the fluid may be trapped on the fiber web(s). In filter elements that include multiple fiber webs, different web(s) may be designed to perform different functions and, thus, may have different characteristics and properties. For example, the filter element may include a fiber web that is primarily designed to filter contaminants (e.g., an efficiency web), and another fiber web that is primarily designed to provide mechanical integrity (e.g., a backing).
Permeability relates to the ability of a fiber web to transmit fluid. It can be expressed as the volume of fluid per unit of time that passes through a unit area of a fiber web at a fixed differential pressure. Filter elements that include multiple fiber webs may include webs having different permeability. For example, web(s) that provide mechanical integrity (e.g., a backing), but are generally not relied on for filtering performance, may be designed to have a high permeability so that such web(s) do not lower the overall permeability of the filter media.
Fiber web(s) can be folded to include sharp, well-defined pleats that increase the fiber web surface area which can increase overall efficiency and performance. Fiber webs that do not pleat well may form rounded, non-uniform pleats, when folded. Such non-uniformity may crowd the space between pleats which ultimately restricts air flow, resulting in lowered efficiency and increased resistance to fluid flow across the fiber web. Several fiber web properties can contribute to its ability to pleat well including stiffness and thickness. In general, it may be difficult to pleat fiber web(s) having a high permeability and a low thickness since such web(s) typically may not have a sufficient stiffness, amongst other factors.